Since the first experiment demonstrating attosecond (as) pulses from High-order Harmonic Generation (HHG) in 2001, the evolution of time resolved research of ultrafast phenomena with the precision of attosecond time scale has greatly impacted scientific understanding of electron dynamic in atoms, molecules and condensed matters. When an intense, few-cycle laser pulse (driver, pump, excitation), usually with the Infrared (IR) spectra (including Near-IR (NIR) and Mid-IR (MIR)), is tightly focused on a gaseous or other suitable target, a bright and coherent laser source in the extreme ultraviolet (XUV) or soft x-ray (SXR) spectra can be generated. This generated XUV/SXR beam can have a broad bandwidth that is several to hundreds of orders of the driving pulse frequency, and a pulse duration on the order of attoseconds (as), representing some of the shortest pulse durations ever created. The generated XUV/SXR beam, however, co-propagates with the residual driving (pump, excitation) pulse.
The practical and effective use of this ultra-short XUV/SXR beam requires the separation of the co-propagating ‘signal’ (XUV/SXR) beam and residual ‘pump’ beam, in large part due to the low (10−5 to 10−6) efficiency of the HHG process that generates the signal beam. This co-propagating beam separation presents a considerable challenge, for example, when a several hundred millijoule (mJ) pump beam is used to generate a high flux signal or when a Mid-IR laser is used for extreme broadband signal spectra generation.
Several exemplary techniques and related apparatus are known, such as thin film metallic filters, the use of annular pump beams, dichroic beam splitters, a Si/SiC plate set at Brewster angle, and a grating beam separator. Their various shortcomings are explained below.
The use of a thin film (hundreds of nanometer thickness) of metallic filter is a straightforward way to block the pump beam while transmitting the XUV/SXR signal beam. However, certain types of metallic filter are transparent only over a certain spectral range. For example, a Molybdenum (Mo) filter only transmits the XUV/SXR beam above 80 electron volts (eV) photon energy. As such, there is an absence of thin film metallic filters that are transparent in a different, desired spectral range. For instance, many pump-probe experiments studying molecular dynamics involve using a photon energy around 10-20 eV; however, no single thin film metallic filter can cover this energy range. Thin film metallic filters also are extremely fragile and some types, such as Zirconium, are very susceptible to strong pump lasers.
The use of annular pump beams, while not susceptible to damage, is very inefficient because a significant among of pump energy is eliminated.
Another approach is to use a dichroic mirror coated with multiple layers of specially chosen materials for best reflectivity. However, the coating design and manufacture is complicated and the high reflectivity can only be obtained within a limited energy range.
A Si/SiC plate at the Brewster angle has been used to separate XUV and IR wavelengths. While this approach provides a high damage threshold and reflectivity higher than 50%, the achievable high reflectivity can only extend to ˜60-70 eV, and more importantly, this method does not work for a Mid-IR pump laser.
Most recently, gratings have been proposed for use as the beam separator. The system can be designed for any wavelength, and the damage threshold is high due to the grazing incident angle on the grating. However, besides involving a complicated system and exhibiting overall low efficiency, different wavelength components of the output XUV/SXR signal beam will be spatially separated. Therefore the capability to spatially overlap all the wavelength components for broadband application remains unproven.
In view of the aforementioned problems, challenges and shortcomings in the art, the inventors have recognized the benefits and advantages that would be provided by solutions embodied in systems, components, and methods for, and utilizing, optical filtering between two different wavelengths or spectra such as, but not necessarily limited to, an IR pump beam and an XUV/SXR signal beam. The benefits and advantages are especially acknowledged for systems, components, and methods for, and utilizing, optical filtering between an IR pump beam having an energy equal to or greater than about 10 mJ and the generated signal beam over an uninterrupted photon energy range from 10 eV to higher than 100 eV, and from 10 eV to higher than 200 eV.